Spread spectrum signals have been utilized in the past for secure communications due to the pseudo-random coding employed and the fact that the spread out signals are barely above the noise level. Because of the spread spectrum and the manner in which that signal is generated, it is difficult to either detect the presence of this spread spectrum signal or to demodulate the signal without foreknowledge of the particular pseudo-random phase shift code.
One type of spread spectrum signal is called a "direct sequence" spread spectrum signal. This signal is generated by rapidly changing the phase of a narrowband signal from 0.degree. to 180.degree. in a pseudo-random known fashion. The effect of pseudo-randomly varying the phase of the signal is to spread out the frequency spectrum of the original signal in a sin x/x fashion. The pseudo-random phase changes are coded such that a predetermined series of phase changes are made to occur, with both the sender and the recipient knowing the code. The code might, for instance, involve shifting from a phase of 0.degree. to 180.degree. at a time T.sub.1 and then shifting back to 0.degree. phase 1 microsecond later; with a further phase shift to 180.degree. 3 microseconds later, etc.
It will be noted that the spread in frequency of the signal is such that the spread spectrum bandwidth is 1/minimum code length. In the above example, the minimum code length would be 1 microsecond and the signal is therefore spread out to a bandwidth of 1 MHz.
What is accomplished by spreading out the signal in this manner is to make the signal appear just above the noise level. For an individual knowing the exact pseudo-random code, the individual can set his receiver so as to change phase exactly in time with the transmitted signal. Once having done this, the pseudo-random phase changes are cancelled out which results in the signal being brought out of the noise. The recipient can then demodulate the remaining signal, assuming that the modulation rate is much slower than the pseudo-random code rate.
It will be appreciated that the direct sequence spread spectrum technique is utilized so as to defeat those who would wish to listen in on the signal. It has been found that it ispossible to both detect the presence and center frequency of a spread spectrum signal and to demodulate the signal without foreknowledge of the code or code correlation techniques. This involves squaring to cancel the pseudo-random code and restore the original signal. Once having squared the incoming signal, part of the invention is the use of compressive receiver technology to detect the presence of spread spectrum signals and their center frequency. Once having detected the center frequency, demodulation is accomplished after narrowband filtering is set to the detected center frequency. A further part of the subject invention involves techniques for rejecting narrowband high power interfering signals by dividing up the frequency band of interest into bins and by hard limiting the signals in each bin, followed by summation of all of the hard limited signals.